V. Megalooikonomou Concurrency Control – Deadlocks (based on slides by C. Faloutsos at CMU and on notes by Silberchatz,Korth, and Sudarshan) Temple University – CIS Dept. CIS616– Principles of Data Management

Isolation - concurrency control serializability correctness precedence graph automatically correct interleavings: locks + protocol (2PL, 2PLC) but: deadlocks!

Deadlocks detection handling (prevention)

Deadlock detection T1 T2 lock manager L(A) Yes L(B) Yes... L(B) No L(A) No time DEADLOCK

Algorithm for deadlock detection? wait-for graph: nodes  transactions arcs  Tsource waits for Tsink if cycle, then deadlock! Must invoke a deadlock-detection algorithm periodically to look for cycles

Example T1 T2 lock manager L(A) Yes L(B) Yes... L(B) No L(A) No time T1T2 ‘A’ ‘B’ for ‘B’ for ‘A’

Another example T1T2 T3 T4 is there a deadlock? if yes, which xacts are involved?

Another example T1T2 T3 T4 now, is there a deadlock? if yes, which xacts are involved?

Deadlock detection how often should we run the algorithm? how many transactions are typically involved?

Deadlock handling T1T2 T3 T4 Q: what to do?

Deadlock handling T1T2 T3 T4 Q0: what to do? A: select a ‘victim’ & ‘rollback’ Q1: which/how to choose?

Deadlock handling Q1: which/how to choose? A1.1: by age A1.2: by progress A1.3: by # items locked already... A1.4: by # xacts to rollback Q2: How far to rollback? T1T2 T3 T4

Deadlock handling Q2: How far to rollback? A2.1: completely A2.2: minimally Q3: Starvation?? T1T2 T3 T4

Deadlock handling Q3: Starvation?? A3.1: include #rollbacks in victim selection criterion T1T2 T3 T4

SQL statement usually, conc. control is transparent to the user, but LOCK [EXCLUSIVE|SHARED]

Concurrency control - conclusions serializability correctness automatically correct interleavings: locks + protocol (2PL, 2PLC,...) deadlock detection + handling

Conclusions serializable schedules 2PL schedules serializable schedules serial sch’s2PLC